A portable information device which is configured so as to be lightweight and compact in consideration of portability is limited in the functions thereof in many cases. Also in a Laptop PC, for example, a reduction in weight or thickness is achieved by limiting the functions in some cases. To such a Laptop PC, a docking device is provided as a dedicated device for extending the functions. For example, a docking device proposed in Japanese Patent No. 6025890 extends the functions by being connector-connected to the Laptop PC.